Hi,
with this little stack you can send a FCM push notification without the need for any middleware (i self use middleware and Pierre helped me with it).
When setting up everything of course you need some PHP and a DB to receive the tokens from your app users.
But this is just simple stack to test when you already know a device token.
This uses the legacy FCM with which you can send to multiple devices in one go, but for now just a single device.
There is also a newer httpv1 API with which you can send to a single device only, but you can distinct Android, ios and web. And you can make subjects where a user can register too etc...